This invention relates to porous metal articles having tailored porosity and methods of manufacturing such articles by using extractable particulates.
Porous metal articles are used in many applications including orthopedic implants, bone growth media, filters, sound suppression materials, fuel cells, catalyst supports, and magnetic shielding. Such porous articles may have open or closed porosity as well as a wide range of pore size, shape, density, and distribution. The specific structures and properties required depend on the application. Known methods of manufacturing porous metals include formation by foaming, diffusion bonding or sintering of powders, depositing a material upon a porous substrate, use of vaporizing materials, and plasma etching.
Foaming describes the creation of porosity through the introduction of some agent, organic or inorganic, that creates voids during the forming process. Diffusion bonding or sintering processes create partially dense networks of powder particles and pores. Porous metal can be formed by the deposition of the desired metal onto a foam or substrate material. This deposition may be accomplished by a number of methods including dipping the substrate material in slurry containing metal powder, evaporation and condensation of the material on the substrate.
When using a vaporizing material as a void former in metals, the material is removed thermally, and the metal matrix material can easily become contaminated by the vaporizing material when the article is exposed to the necessary high processing temperatures. The evolution of the products of decomposition and the vaporized materials are difficult to control which reduces the robustness of the process, thus limiting processing capabilities to articles having smaller cross-sections. Complications resulting from oxidation and other contamination in thermal decomposition or sublimation approaches make such processes not suitable for high-purity applications such as bone implants. In addition, the heat and high pressure of the compaction and densification processes will deform the void former material, thereby not allowing the tailoring of pore properties.
The pores formed by the plasma spray process are limited in the thickness of the pore layer and result in random pore formation. Vapor deposition techniques for making porous tantalum structures results in articles having a relatively poor bending strength and not suited in many applications where the article has bending force exerted thereon.
Moreover, techniques depositing a powder onto a foam substrate via slurry are limited by the slurry's ability to penetrate and evenly coat the substrate. Other methods such the construction of shapes by the laser sintering of powders are limited to small section by the inherent expense and time-intensive nature of the process.
Thus, there remains a need to provide a porous metal with a controllable pore structure which possesses strength and structural integrity and is free of contamination.